Planetary gear train of automatic transmission for vehicles

ABSTRACT

A planetary gear train may include an input shaft, an output shaft, a first planetary gear set, a second planetary gear set, a first rotation shaft directly connected to one rotation element of the first planetary gear set and one rotation element of the second planetary gear set, and selectively connected to the input shaft, a second rotation shaft directly connected to another rotation element of the second planetary gear set and directly connected to the output shaft, a third rotation shaft directly connected to another rotation element of the first planetary gear set and the other rotation element of the second planetary gear set, and selectively connected to the input shaft or to a transmission housing, and a fourth rotation shaft directly connected to the other rotation element of the first planetary gear set, and selectively connected to the input shaft through two paths or to the transmission housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of KoreanPatent Application No. 10-2013-0145217 filed on Nov. 27, 2013, theentire contents of which is incorporated herein for all purposes by thisreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that can improvepower delivery performance and reduce fuel consumption at a low-speeddriving.

2. Description of Related Art

Typically, multiple-shift mechanism of an automatic transmission isachieved by combining a plurality of planetary gear sets and a pluralityof frictional elements. It is well known that when a planetary geartrain realizes a greater number of shift speeds, speed ratios of theplanetary gear train can be more optimally designed, and therefore avehicle can have economical fuel mileage and better performance. Forthat reason, the planetary gear train that is able to realize more shiftspeeds is under continuous investigation.

Though achieving the same number of speeds, the planetary gear train hasa different operating mechanism according to a connection betweenrotation elements (i.e., sun gear, planet carrier, and ring gear). Inaddition, the planetary gear train has different features such adurability, power delivery efficiency, and size depend on the layoutthereof. Therefore, designs for a combining structure of a gear trainare also under continuous investigation.

If the number of shift-speeds, however, increases, the number ofcomponents in the automatic transmission also increases. Therefore,mountability, cost, weight and power delivery efficiency may bedeteriorated.

Particularly, since the planetary gear train having a number ofcomponents is hard to be mounted in a front wheel drive vehicle,researches for minimizing the number of components have been developed.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle havingadvantages of achieving eight forward speeds and one reverse speed bydisposing first and second planetary gear sets on an output shaft andconnecting rotation elements of the first and second planetary gear setsto an input shaft through a plurality of transfer gears.

In addition, various aspects of the present invention are directed toproviding a planetary gear train of an automatic transmission for avehicle having further advantages of enabling of setting optimum gearratios due to ease of changing gear ratios by using the plurality oftransfer gears, and accordingly improving power delivery performance andfuel economy at a low-speed driving.

A planetary gear train of an automatic transmission for a vehicleaccording to an exemplary embodiment of the present invention mayinclude, an input shaft receiving torque, an output shaft disposed inparallel with and apart from the input shaft, a first planetary gear sethaving three rotation elements, a second planetary gear set having threerotation elements, a first rotation shaft directly connected to onerotation element of the first planetary gear set and one rotationelement of the second planetary gear set, and selectively connected tothe input shaft, a second rotation shaft directly connected to anotherrotation element of the second planetary gear set and directly connectedto the output shaft. a third rotation shaft directly connected toanother rotation element of the first planetary gear set and the otherrotation element of the second planetary gear set, and selectivelyconnected to the input shaft or to a transmission housing, and, a fourthrotation shaft directly connected to the other rotation element of thefirst planetary gear set, and selectively connected to the input shaftthrough two paths or to the transmission housing.

The planetary gear train may further include three transfer gearsdisposed respectively between the input shaft and the first and thirdrotation shafts and on the two paths.

The planetary gear train may further include a dog clutch selectivelytransmitting torque of the input shaft to any one of the two paths ornot transmitting either of the two paths.

Each of the first and second planetary gear sets may be a single pinionplanetary gear set.

The first rotation shaft may be connected to a first sun gear of thefirst planetary gear set and a second sun gear of the second planetarygear set, the second rotation shaft may be connected to a second planetcarrier of the second planetary gear set, the third rotation shaft maybe connected to a first planet carrier of the first planetary gear setand a second ring gear of the second planetary gear set, and the fourthrotation shaft may be connected to a first ring gear of the firstplanetary gear set.

The three transfer gears may include, a first transfer gear disposedbetween the input shaft and the first and third rotation shafts, asecond transfer gear disposed on a first path of the two paths, and athird transfer gear disposed on a second path of the two paths.

Gear ratios of the three transfer gears may be different from eachother.

The planetary gear train may further include, a first clutch disposedbetween the first transfer gear and the first rotation shaft, a secondclutch disposed between the first transfer gear and the third rotationshaft, a first brake disposed between the third rotation shaft and thetransmission housing, and. a second brake disposed between the fourthrotation shaft and the transmission housing.

The first, second and third transfer gears and the first and secondclutches may be disposed at one side of the first planetary gear set,and the first and second brakes may be disposed at the other side of thefirst planetary gear set.

A planetary gear train of an automatic transmission for a vehicleaccording to another exemplary embodiment of the present invention mayinclude, an input shaft receiving torque, an output shaft disposed inparallel with and apart from the input shaft, a first planetary gear setincluding a first sun gear, a first planet carrier, and a first ringgear as rotation elements thereof, a second planetary gear set includinga second sun gear, a second planet carrier, and a second ring gear asrotation elements thereof and disposed adjacent to the first planetarygear set, a first rotation shaft directly connected to the first sungear and the second sun gear and selectively connected to the inputshaft, a second rotation shaft directly connected to the second planetcarrier and directly connected to the output shaft. a third rotationshaft directly connected to the first planet carrier and the second ringgear, and selectively connected to the input shaft or to a transmissionhousing. a fourth rotation shaft directly connected to the first ringgear and selectively connected to the input shaft or to the transmissionhousing, and a dog clutch selectively connecting the input shaft to thefourth rotation shaft through two paths.

Each of the first and second planetary gear sets may be a single pinionplanetary gear set.

The planetary gear train may further include, a first transfer geardisposed between the input shaft and the first and third rotationshafts, a second transfer gear disposed on a first path of the twopaths, and a third transfer gear disposed on a second path of the twopaths.

Gear ratios of the three transfer gears may be different from eachother.

The planetary gear train may further include, a first clutch disposedbetween the first transfer gear and the first rotation shaft. a secondclutch disposed between the first transfer gear and the third rotationshaft, a first brake disposed between the third rotation shaft and thetransmission housing, and a second brake disposed between the fourthrotation shaft and the transmission housing.

The first, second and third transfer gears and the first and secondclutches may be disposed at one side of the first planetary gear set,and the first and second brakes may be disposed at the other side of thefirst planetary gear set.

The dog clutch may be adapted to selectively transmit torque of theinput shaft to any one of the two paths or not to transmit either of thetwo paths.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

FIG. 2 is a drawing for illustrating operation of a dog clutch used in aplanetary gear train according to an exemplary embodiment of the presentinvention.

FIG. 3 is an operational chart of friction elements at each shift-speedapplied to a planetary gear train according to an exemplary embodimentof the present invention.

FIG. 4 is a lever diagram of a planetary gear train according to anexemplary embodiment of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

An exemplary embodiment of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

Description of components that are not necessary for explaining thepresent exemplary embodiment will be omitted, and the same constituentelements are denoted by the same reference numerals in thisspecification.

In the detailed description, ordinal numbers are used for distinguishingconstituent elements having the same terms, and have no specificmeanings.

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplaryembodiment of the present invention includes an input shaft IS receivingtorque of an engine, an output shaft OS disposed in parallel with andapart from the input shaft IS, first and second planetary gear sets PG1and PG2 disposed on the output shaft OS, three transfer gears TF1, TF2,and TF3, a dog clutch DC, and frictional elements including two clutchesC1 and C2 and two brakes B1 and B2.

Therefore, torque input from the input shaft IS is converted into eightforward speeds and one reverse speed by cooperation of the first andsecond planetary gear sets PG1 and PG2, and is then output through theoutput shaft OS.

The input shaft IS is an input member and receives torque from acrankshaft of the engine.

The output shaft OS drives a driving shaft including a driving wheelthrough a final reduction gear and a differential apparatus.

The first and second planetary gear sets PG1 and PG2 are disposed on theoutput shaft OS and are combined with each other to include fourrotation shafts N1, N2, N3, and N4.

The first planetary gear set PG1 is a single pinion planetary gear set,and includes a first sun gear S1, a first planet carrier PC1 rotatablysupporting a first pinion P1 externally meshed with the first sun gearS1, and a first ring gear R1 internally meshed with as rotation elementsthereof.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 externally meshed with the second sun gearS2, and a second ring gear R2 internally meshed with the second pinionP2 as rotation elements thereof.

The first sun gear S1 is directly connected to the second sun gear S2,the first planet carrier PC1 is directly connected to the second ringgear R2 such that the first and second planetary gear sets PG1 and PG2are operated as a compound planetary gear set including four rotationshafts N1, N2, N3, and N4.

The first rotation shaft N1 is directly connected to the first sun gearS1 and the second sun gear S2, and is selectively connected to the inputshaft IS.

The second rotation shaft N2 is directly connected to the second planetcarrier PC2 and is directly connected to the output shaft OS to bealways operated as an output element.

The third rotation shaft N3 is directly connected to the first planetcarrier PC1 and the second ring gear R2, and is selectively connected tothe input shaft IS or to a transmission housing H.

The fourth rotation shaft N4 is directly connected to the first ringgear R1 and is selectively connected to the input shaft IS through twopaths or to the transmission housing H.

The first, second, and third transfer gears TF1, TF2, and TF3respectively have first, second, and third transfer drive gears TF1a,TF2a, and TF3a and first, second, and third transfer driven gears TF1b,TF2b, and TF3b externally meshed with each other.

In addition, gear ratios of the first, second and third transfer gearsTF1, TF2, and TF3 may be set according to target speed ratios. Forexample, the gear ratio of the second transfer gear TF2 is greater thanthat of the first transfer gear TF1 and is smaller than that of thethird transfer gear TF3.

Connections of the first, second, and third transfer gears TF1, TF2, andTF3 will be described in detail.

The first transfer gear TF1 connects the first rotation shaft N1 and thethird rotation shaft N3 to the input shaft IS. That is, the firsttransfer drive gear TF1a is connected to the input shaft IS and thefirst transfer driven gear TF1b is connected to the first rotation shaftN1 and the third rotation shaft N3.

The second transfer gear TF2 connects the fourth rotation shaft N4 tothe input shaft IS. That is, the second transfer drive gear TF2a isconnected to the input shaft IS and the second transfer driven gear TF2bis connected to the fourth rotation shaft N4.

The third transfer gear TF3 connects the fourth rotation shaft N4 to theinput shaft IS. That is, the third transfer drive gear TF3a is connectedto the input shaft IS and the third transfer driven gear TF3b isconnected to the fourth rotation shaft N4.

The dog clutch DC is widely used in a manual transmission and is wellknown to a person of an ordinary skill in the art. The dog clutch DCselectively transmits torque of the input shaft IS to the secondtransfer gear TF2 or the third transfer gear TF3, or does not transmitthe torque of the input shaft IS to either of the second and the thirdtransfer gears TF2 and TF3.

In addition, two clutches C1 and C2 connecting selected rotation shaftsto the input shaft IS and two brakes B1 and B2 connecting selectedrotation shafts to the transmission housing H are disposed as follows.

The first clutch C1 is disposed between the first transfer driven gearTF1b of the first transfer gear TF1 and the first rotation shaft N1.

The second clutch C2 is disposed between the first transfer driven gearTF1b of the first transfer gear TF1 and the third rotation shaft N3.

The first brake B1 is disposed between the third rotation shaft N3 andthe transmission housing H.

The second brake B2 is disposed between the fourth rotation shaft N4 andthe transmission housing H.

The frictional elements including the first and second clutches C1 andC2 and the first and second brakes B1 and B2 are conventionalmulti-plate friction elements of wet type that are operated by hydraulicpressure.

The first, second, and third transfer gears TF1, TF2, and TF3 aredisposed at one side of the first planetary gear set PG1.

In addition, the first and second clutches C1 and C2 are disposed at theone side of the first planetary gear set PG1, and the first and secondbrakes B1 and B2 are disposed at the other side of the first planetarygear set PG1.

FIG. 2 is a drawing for illustrating operation of a dog clutch used in aplanetary gear train according to an exemplary embodiment of the presentinvention.

Referring to FIG. 2, the dog clutch DC includes a dog clutch hub DCH, adog clutch sleeve DCS, and first and second dog clutch gears DCG1 andDCG2.

The dog clutch hub DCH is directly connected to the input shaft IS andalways receive the torque of the input shaft IS.

The dog clutch sleeve DCS is splined to an exterior circumferentialportion of the dog clutch hub DCH to be slidable in an axial direction,and moves in the axial direction by an actuator.

The first and second dog clutch gears DCG1 and DCG2 are directlyconnected respectively to the second and third transfer gears TF2 andTF3. The first and second dog clutch gears DCG1 and DCG2 are selectivelyengaged with the dog clutch sleeve DCS and selectively receive thetorque of the input shaft IS.

Since the dog clutch DC is well known to a person of an ordinary skillin the art, detailed description thereof will be omitted.

FIG. 2A illustrates the dog clutch DC at a neutral state. As shown inFIG. 2A, the dog clutch sleeve DCS is poisoned at the center and is notengaged to neither of the first and second dog clutch gears DCG1 andDCG2 at the neutral state.

FIG. 2B illustrates that the torque of the input shaft IS is transmittedto the second transfer gear TF2. As shown in FIG. 2B, when the dogclutch sleeve DCS moves to the left in the drawings by the actuator, thedog clutch hub DCH and the first dog clutch gear DCG1 are operablyconnected.

In this case, the torque of the input shaft IS is transmitted to thesecond transfer gear TF2 through the dog clutch DC.

FIG. 2C illustrates that the torque of the input shaft IS is transmittedto the third transfer gear TF3. As shown in FIG. 2C, when the dog clutchsleeve DCS moves to the right in the drawings by the actuator, the dogclutch hub DCH and the second dog clutch gear DCG2 are operablyconnected.

In this case, the torque of the input shaft IS is transmitted to thethird transfer gear TF3 through the dog clutch DC.

FIG. 3 is an operational chart of friction elements at each shift-speedapplied to a planetary gear train according to an exemplary embodimentof the present invention.

Referring to FIG. 3, two frictional elements or one frictional elementand the dog clutch DC are operated at each shift-speed according to theexemplary embodiment of the present invention.

The first clutch C1 and the first brake B1 are operated and the dogclutch DC stays at the neutral state at a first forward speed 1ST.

The first clutch C1 and the second brake B2 are operated and the dogclutch DC stays at the neutral state at a second forward speed 2ND.

The first clutch C1 is operated and the dog clutch DC is operablyconnected to the third transfer gear TF3 at a third forward speed 3RD.

The first clutch C1 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a fourth forward speed 4TH.

The first clutch C1 and the second clutch C2 are operated and the dogclutch DC stays at the neutral state at a fifth forward speed 5TH.

The second clutch C2 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a sixth forward speed 6TH.

The second clutch C2 is operated and the dog clutch DC is operablyconnected to the third transfer gear TF3 at a seventh forward speed 7TH.

The second clutch C2 and the second brake B2 are operated and the dogclutch DC stays at the neutral state at an eighth forward speed 8TH.

The first brake B1 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a reverse speed REV.

FIG. 4 is a lever diagram of a planetary gear train according to theexemplary embodiment of the present invention and illustrates shiftprocesses of the planetary gear train according to the exemplaryembodiment of the present invention by lever analysis method.

Referring to FIG. 4, four vertical lines of the first and secondplanetary gear sets PG1 and PG2 are set as the first, second, third, andfourth rotation shafts N1, N2, N3, and N4 from the left to the right. Inaddition, a lower horizontal line represents a rotation speed of “0”, anupper portion of the horizontal line represents positive rotation speed,and a lower portion of the horizontal line represents negative rotationspeed.

At this time, although rotation speed input to each rotation shaft isnegative rotation speed, for better comprehension and ease ofdescription, it is illustrates that positive rotation speed is input toeach rotation shaft.

Hereinafter, referring to FIG. 3 and FIG. 4, the shift processes of theplanetary gear train according to the exemplary embodiment of thepresent invention will be described in detail.

[First Forward Speed]

Referring to FIG. 3, the first clutch C1 and the first brake B1 areoperated and the dog clutch DC stays at the neutral state at the firstforward speed 1ST.

As shown in FIG. 4, since the dog clutch DC stays at the neutral state,the torque of the input shaft IS is not input to the fourth rotationshaft N4 and is input to the first rotation shaft N1 through the firsttransfer gear TF1 by operation of the first clutch C1. In addition, thethird rotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a first shift line SP1 and D1 isoutput through the second rotation shaft N2 that is the output element.

[Second Forward Speed]

The first brake B1 that was operated at the first forward speed 1ST isreleased and the second brake B2 is operated at the second forward speed2ND.

In this case, since the dog clutch DC stays at the neutral state, thetorque of the input shaft IS is not input to the fourth rotation shaftN4 and is input to the first rotation shaft N1 through the firsttransfer gear TF1 by operation of the first clutch C1. In addition, thefourth rotation shaft N4 is stopped by operation of the second brake B2.

Therefore, the rotation shafts form a second shift line SP2 and D2 isoutput through the second rotation shaft N2 that is the output element.

[Third Forward Speed]

The second brake B2 that was operated at the second forward speed 2ND isreleased and the dog clutch DC is operably connected to the thirdtransfer gear TF3 at the third forward speed 3RD.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 through the first transfer gear TF1 by operation ofthe first clutch C1 and is input to the fourth rotation shaft N4 throughthe third transfer gear TF3.

Therefore, the rotation shafts form a third shift line SP3 by the gearratios of the first transfer gear TF1 and the third transfer gear TF3,and D3 is output through the second rotation shaft N2 that is the outputelement.

[Fourth Forward Speed]

If the dog clutch DC is released from the third transfer gear TF3 and isoperably connected to the second transfer gear TF2 at the third forwardspeed 3RD, the fourth forward speed 4TH is achieved.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 through the first transfer gear TF1 by operation ofthe first clutch C1 and is input to the fourth rotation shaft N4 throughthe second transfer gear TF2.

Therefore, the rotation shafts form a fourth shift line SP4 by the gearratios of the first transfer gear TF1 and the second transfer gear TF2,and D4 is output through the second rotation shaft N2 that is the outputelement.

[Fifth Forward Speed]

If the dog clutch DC returns to the neutral state and the second clutchC2 is operated at the fourth forward speed 4TH, the fifth forward speed5TH is achieved.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 and the third rotation shaft N3 through the firsttransfer gear TF1 by operation of the first clutch C1 and the secondclutch C2.

Therefore, the first and second planetary gear sets PG1 and PG2 becomedirect-coupling state, the rotation shafts form a fifth shift line SP5,and D5 is output through the second rotation shaft N2 that is the outputelement.

[Sixth Forward Speed]

The first clutch C1 that was operated at the fifth forward speed 5TH isreleased and the dog clutch DC is operably connected to the secondtransfer gear TF2 at the sixth forward speed 6TH.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2 and is input to the fourth rotation shaft N4through the second transfer gear TF2.

Therefore, the rotation shafts form a sixth shift line SP6 by the gearratios of the first transfer gear TF1 and the second transfer gear TF2,and D6 is output through the second rotation shaft N2 that is the outputelement.

[Seventh Forward Speed]

If the dog clutch DC is released from the second transfer gear TF2 andis operably connected to the third transfer gear TF3 at the sixthforward speed 6TH, the seventh forward speed 7TH is achieved.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2 and is input to the fourth rotation shaft N4through the third transfer gear TF3.

Therefore, the rotation shafts form a seventh shift line SP7 by the gearratios of the first transfer gear TF1 and the third transfer gear TF3,and D7 is output through the second rotation shaft N2 that is the outputelement.

[Eighth Forward Speed]

If the dog clutch DC returns to the neutral state and the second brakeB2 is operated at the seventh forward speed 7TH, the eighth forwardspeed 8TH is achieved.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2, and the fourth rotation shaft N4 is stopped byoperation of the second brake B2.

Therefore, the rotation shafts form an eighth shift line SP8 and D8 isoutput through the second rotation shaft N2 that is the output element.

[Reverse Speed]

The first brake B1 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at the reverse speed REV.

In this case, the torque of the input shaft IS is input to the fourthrotation shaft N4 through the second transfer gear TF2, and the thirdrotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a reverse shift line RS and REV isoutput through the second rotation shaft N2 that is the output element.

The planetary gear train according to the exemplary embodiment of thepresent invention can achieve eight forward speeds and one reverse speedby combining two planetary gear sets PG1 and PG2 being the simpleplanetary gear sets with three transfer gears TF1, TF2, and TF3 beingthe externally-meshed gears, four frictional elements C1, C2, B1, andB2, and the dog clutch DC.

Since the first, second, and third transfer gears TF1, TF2, and TF3 andthe dog clutch DC are used, at least one planetary gear set and twofrictional elements may be removed, compared with the planetary geartrain. Therefore, manufacturing cost may be curtained by reducing thenumber of components.

In addition, optimum gear ratios may be set due to ease of changing gearratios by using the first, second, and third transfer gears TF1, TF2,and TF3 as well as the first and second planetary gear sets PG1 and PG2.Since the gear ratios can be changed according to target performance,starting performance, power delivery performance and fuel economy may beimproved.

In addition, two frictional elements or one frictional element and thedog clutch are operated at each shift-speed, and one frictional elementis released and another friction element is operated or only the dogclutch changes its operating state to shift to a neighboringshift-speed. Therefore, shift control condition is fully satisfied.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings as well as various alternatives and modifications thereof. Itis intended that the scope of the invention be defined by the Claimsappended hereto and their equivalents.

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle comprising: an input shaft receiving torque;an output shaft disposed in parallel with and apart from the inputshaft; a first planetary gear set having three rotation elements; asecond planetary gear set having three rotation elements; a firstrotation shaft directly connected to one rotation element of the firstplanetary gear set and one rotation element of the second planetary gearset, and selectively connected to the input shaft; a second rotationshaft directly connected to another rotation element of the secondplanetary gear set and directly connected to the output shaft; a thirdrotation shaft directly connected to another rotation element of thefirst planetary gear set and the other rotation element of the secondplanetary gear set, and selectively connected to the input shaft or to atransmission housing; and a fourth rotation shaft directly connected tothe other rotation element of the first planetary gear set, andselectively connected to the input shaft through two paths or to thetransmission housing.
 2. The planetary gear train of claim 1, furthercomprising three transfer gears disposed respectively between the inputshaft and the first and third rotation shafts and on the two paths. 3.The planetary gear train of claim 1, further comprising a dog clutchselectively transmitting torque of the input shaft to any one of the twopaths or not transmitting either of the two paths.
 4. The planetary geartrain of claim 1, wherein each of the first and second planetary gearsets is a single pinion planetary gear set.
 5. The planetary gear trainof claim 1, wherein the first rotation shaft is connected to a first sungear of the first planetary gear set and a second sun gear of the secondplanetary gear set, the second rotation shaft is connected to a secondplanet carrier of the second planetary gear set, the third rotationshaft is connected to a first planet carrier of the first planetary gearset and a second ring gear of the second planetary gear set, and thefourth rotation shaft is connected to a first ring gear of the firstplanetary gear set.
 6. The planetary gear train of claim 2, wherein thethree transfer gears comprise: a first transfer gear disposed betweenthe input shaft and the first and third rotation shafts; a secondtransfer gear disposed on a first path of the two paths; and a thirdtransfer gear disposed on a second path of the two paths.
 7. Theplanetary gear train of claim 2, wherein gear ratios of the threetransfer gears are different from each other.
 8. The planetary geartrain of claim 6, further comprising: a first clutch disposed betweenthe first transfer gear and the first rotation shaft; a second clutchdisposed between the first transfer gear and the third rotation shaft; afirst brake disposed between the third rotation shaft and thetransmission housing; and a second brake disposed between the fourthrotation shaft and the transmission housing.
 9. The planetary gear trainof claim 8, wherein the first, second, and third transfer gears and thefirst and second clutches are disposed at one side of the firstplanetary gear set, and the first and second brakes are disposed at theother side of the first planetary gear set.
 10. A planetary gear trainof an automatic transmission for a vehicle comprising: an input shaftreceiving torque; an output shaft disposed in parallel with and apartfrom the input shaft; a first planetary gear set including a first sungear, a first planet carrier, and a first ring gear as rotation elementsthereof; a second planetary gear set including a second sun gear, asecond planet carrier, and a second ring gear as rotation elementsthereof and disposed adjacent to the first planetary gear set; a firstrotation shaft directly connected to the first sun gear and the secondsun gear and selectively connected to the input shaft; a second rotationshaft directly connected to the second planet carrier and directlyconnected to the output shaft; a third rotation shaft directly connectedto the first planet carrier and the second ring gear, and selectivelyconnected to the input shaft or to a transmission housing; a fourthrotation shaft directly connected to the first ring gear and selectivelyconnected to the input shaft or to the transmission housing; and a dogclutch selectively connecting the input shaft to the fourth rotationshaft through two paths.
 11. The planetary gear train of claim 10,wherein each of the first and second planetary gear sets is a singlepinion planetary gear set.
 12. The planetary gear train of claim 10,further comprising: a first transfer gear disposed between the inputshaft and the first and third rotation shafts; a second transfer geardisposed on a first path of the two paths; and a third transfer geardisposed on a second path of the two paths.
 13. The planetary gear trainof claim 12, wherein gear ratios of the three transfer gears aredifferent from each other.
 14. The planetary gear train of claim 13,further comprising: a first clutch disposed between the first transfergear and the first rotation shaft; a second clutch disposed between thefirst transfer gear and the third rotation shaft; a first brake disposedbetween the third rotation shaft and the transmission housing; and asecond brake disposed between the fourth rotation shaft and thetransmission housing.
 15. The planetary gear train of claim 14, whereinthe first, second, and third transfer gears and the first and secondclutches are disposed at one side of the first planetary gear set, andthe first and second brakes are disposed at the other side of the firstplanetary gear set.
 16. The planetary gear train of claim 14, whereinthe dog clutch is adapted to selectively transmit torque of the inputshaft to any one of the two paths or not to transmit either of the twopaths.